Conductive polymer compositions, also called conductive composites or conductive polymer composites, and electrical devices comprising them are well known. Such compositions comprise a polymer and, dispersed in the polymer, a particulate conductive filler. The type and quantity of the conductive particles, as well as the type of the polymer, influence the resistivity of the composition. For compositions with resistivities greater than about 1 ohm-cm, carbon black is a preferred filler. For compositions with lower resistivities, metal particles are used. Compositions comprising carbon black are described, for example, in U.S. Pat. No. 4,237,441 (van Konynenburg et al.), U.S. Pat. No. 4,388,607 (Toy et al.), U.S. Pat. No. 4,534,889 (van Konynenburg et al.), U.S. Pat. No. 4,560,498 (Horsma et al.), U.S. Pat. No. 4,591,700 (Sopory), U.S. Pat. No. 4,724,417 (Au et al.), U.S. Pat. No. 4,774,024 (Deep et al.), U.S. Pat. No. 4,935,156 (van Konynenburg et al.), and U.S. Pat. No. 5,049,850 (Evans et al.). Compositions comprising metal fillers are described, for example, in U.S. Pat. No. 4,545,926 (Fouts et al.), U.S. Pat. No. 5,250,228 (Baigrie et al.), U.S. Pat. No. 5,378,407 (Chandler et al.) and U.S. Pat. No. 7,371,459 (Galla). The disclosure of each of these patents is incorporated herein by reference.
The electrical properties of conductive polymer composites tend to deteriorate over time. For example, in metal-filled conductive polymer composites, the surfaces of the metal particles tend to oxidize when the composite is in contact with an ambient atmosphere, and the resultant oxidation layer reduces the conductivity of the particles when in contact with each other.
Conductive polymer composites are used to make polymeric positive temperature coefficient (PPTC) devices. Such devices, which are often installed in an electrical circuit, increase by orders of magnitude in resistance when exposed to an overtemperature or overcurrent condition. Removal of the overtemperature or overcurrent device and cycling of any power through the circuit allows the device to return to its low resistance, room temperature condition. However, the oxidation that conventional metal particles used as fillers in conductive composites exhibit can cause significant instability in the PPTC device, resulting in an increase in resistance (reduction in conductivity) after each excursion to a high temperature, high resistance state, making the devices unsuitable for prolonged use.
The electrical performance of devices containing conductive polymer composites can be improved by minimizing the exposure of the composite to oxygen, by use, for example, of a protective layer such as an epoxy, a silicone, or an insulating tape to cover some or all of the composite. Alternatively, the use of a particle that is less sensitive to oxidation, e.g. titanium carbide, can provide stability. These approaches have drawbacks in terms of increased manufacturing complexity and/or insufficiently low electrical resistivity.
The use of conductive composites with decreased resistivity and increased conductivity, without sacrificing cost, operational complexity, or operational properties continues to be desirable in the art. Also, having good stability under electrical operating conditions continues to be desirable in the art.